There have been many attempts to raise fish in large quantities for the commercial market. In order to successfully accomplish this, however, very careful control over the certain parameters is required.
For example, the oxygen content of the water used to raise fish is extremely critical. If the oxygen level drops below a minimum level, fish will die. Usually, this minimum level is not far below the saturation level at the temperature of the water employed, which means that any significant depletion of the oxygen present will lower the level below that required.
Water temperature is also crucial. Rainbow trout, for example, seem to grow best at a temperature around the mid-50's (.degree.F.). If the water temperature is raised too far above this value, disease becomes more prevalent. On the other hand, if the temperature is dropped too much below this value, the fish simply do not grow at a rapid enough rate to make the venture commercially viable.
Another serious problem is the accumulation of particulate matter at the bottom of a fish tank, expecially metabolic waste and excess food. Accumulated metabolic waste tends to contaminate the water with its by-products, including ammonia, urea, and other compounds which are toxic for fish at relatively low levels. Leaching of toxic compounds from metabolic waste is aggravated if the waste is subjected to agitation, such as is the case if the water flow in the fish tanks is too rapid or too turbulent.
Food is another variable in fish raising which must be carefully controlled. Food is, of course, expensive and overfeeding tends to take away from the commercial viability of the venture. This is particularly true with many commercial species of fish, such as salmonids, which are principally mid-water feeders. In addition, excess food accumulation also depletes oxygen since oxygen in the water is consumed in the decomposition of excess food. Excess food is simply wasted, consumes oxygen, adds inordinate expense and contributes to disease.
On the other hand, underfeeding is also a problem since it results in a slower growth rate which detracts from the commercial viability of fish raising. Thus, it is extremely important in the commercial raising of fish to provide precisely the proper amount of food, and no more or no less.
Both raceways and tanks have been developed and employed in attempts to raise fish commercially. Raceways are elongated shallow channels through which vast quantities of water are flowed. Raceways provide little or no control, however, over the important variables, and so, their use has been limited to situations in which there is a natural supply of water which just happens to be right for raising fish. Thus, raceways are often employed near mountain streams having water at an appropriate temperature for fish raising.
A variety of tank systems have been designed in attempts to control one or more of these important parameters, and some of these are described in the patent literature. One example, described in U.S. Pat. No. 3,565,043, discloses a fish husbandry system which has an upper fish habitat compartment, with a water supply spray, and a lower control compartment separated by a slanted partition to guide waste matter and pollutants through an adjustable flow hole into the lower compartment. U.S. Pat. No. 3,583,365 describes an apparatus useful in concentrating live fish in a container for transfer to another location. U.S. Pat. No. 3,661,119 discloses a fish larvae rearing tank with a water circulation system for rapidly exchanging the water in the tank which is characterized by the fact that it produces minimum turbulence and air bubbles. U.S. Pat. No. 3,756,197 discloses a generally vertical, waterfilled tank into which oxygen-containing gas is injected under pressure to a hood section which retains the oxygen so that it can be dissolved into the water. U.S. Pat. No. 3,771,492 discloses a system for handling large quantities of water, obtained from wells or springs, which is used to raise fish and which is subsequently returned into the earth.
Another fish raising tank that has been employed has a circular cross-section at its upper portion with sloping walls at its lower portion. The sloping walls are intended to create a whirlpool motion to water flowing through this tank in an effort to entrain particulate material and carry it out of the tank. This shape doesn't approximate a natural fish habitat, however, and has resulted in bunching of fish and an uneven distribution. The swirling action of water in such tanks also tends to agitate particulate matter, including metabolic waste, and this produces a leaching of toxic products which is highly undesirable. In addition, tanks of this type, or any other design requiring the action of rapidly flowing water to remove accumulated particulate matter, requires the flow of vast quantities of water which can create a severe environmental problem.
Applicant's parent application, which issued on Sept. 9, 1980, as U.S. Pat. No. 4,221,187, disclosed a different type of tank apparatus designed for commercial raising of fish. The main tank may be of virtually any size, depending upon the number of fish contained; for example, suitable dimensions might be a rectangular tank 80 ft. long.times.40 ft. wide.times.7 ft. deep. Water is introduced into one end of the fish tank using means for initiating laminar flow. Laminar flow requires the fish to expend only minimum exertion swimming and, yet, eliminates the creation of pockets of non-circulating water. Laminar flow also reduces the quantity of water required compared to other tank or raceway designs.
A particularly unique feature of this apparatus is the sweeper apparatus which is employed. This sweeper apparatus has one or more sweeper heads slidably mounted at the bottom of the tank so that they can move longitudinally across the surface of the tank bottom. The sweeper head assembly is attached to a pump which draws a relatively small quantity of water into the sweeper head and from the tank. This small quantity of water is sufficient to entrain particulate matter which has accumulated at the tank bottom.
The main purpose of this sweeper is to clean particulate matter, such as metabolic waste products and excess food, which has accumulated at the bottom of the fish tank. In addition, the sweeper may be provided with means to introduce oxygen into the tank as well as means to introduce a disinfectant, such as chlorine gas. Although chlorine is highly toxic, the unique sweeper design allows it to be used without contaminating the bulk of the tank.
Water picked up by the sweeper is pumped through an exit line. In previous tank embodiments that did not contain sweeping devices, effluent water containing fish waste and excess food was distributed over a ground area for agricultural purposes, or otherwise disposed of as cheaply as possible. However, the sweeping apparatus disclosed in the parent application, now U.S. Pat. No. 4,221,187, allows for a variety of useful procedures to analyze and reuse the particulate material removed from fish tanks.